1. Field of the Invention
The present invention relates generally to systems for irradiating targets with electromagnetic radiation, and more specifically to annular-type applicators and associated systems for controlling application of radiation to biological tissue.
2. Description of Prior Art
Several types of therapeutic treatments for cancer in humans are in current, common use. These treatments include surgery, X-rays, radiation from radioactive sources and chemotherapy, these treatments being often combined in various ways to enhance treatment effectiveness.
Although such conventional treatment techniques have been successful in treating cancer in many patients and in prolonging the lives of many other patients, they are frequently ineffective against many types of cancer and often have severe adverse side effects at the necessary treatment levels. Protracted treatment of cancer patients by x-rays or chemotherapy, as an illustration, tends to eventually destroy or inhibit the patients' natural immunological systems to an extent that many patients eventually succumb to common infectious diseases, such as influenza or pneumonia, which otherwise probably would not be fatal. Also, many patients having advanced stages of cancer or complications may become too weak to withstand the trauma of surgical or other cancer treatments; hence, the treatments cannot be undertaken or must be discontinued.
Due both to the prevalence and the typically severe consequences of human cancer, as well as frequent ineffectiveness of current treatments such as those mentioned above, medical researchers are continually experimenting in an attempt to discover and develop improved or alternative cancer treatment methods with their associated treatment apparatus.
Hyperthermia, the generation of artificially elevated body temperatures, has recently been given serious scientific consideration as an alternative cancer treatment. Much research has been conducted into the effectiveness of hyperthermia alone or in combination with other treatment methods. This research is important in that hyperthermia techniques appear to have the potential for being extremely effective in the treatment of many or most types of human cancers, without the often severely adverse side effects associated with current cancer treatments.
Researchers into hyperthermia treatment of cancer have commonly reported that many types of malignant growths in humans can be thermally destroyed, usually with no serious adverse side effects, by heating the malignancies to temperatures slightly below that injurious to most normal, healthy cells. Furthermore, many types of malignant cell masses have reportedly been found to have substantially poorer than normal heat transfer or dissipation characteristics, presumably due to poorer vascularity and reduced blood flow characteristics. Consequently, these types of growths appear capable of preferential hyperthermia treatment. Poorly vascular malignant growths can reportedly be heated to temperatures several degrees higher than that in which the immediately surrounding healthy tissue reaches. This promises to enable hyperthermic treatment of those types of malignant growths which are no more thermally sensitive than normal tissue without destruction of normal cells, and additionally to enable higher temperature, shorter hyperthermia treatment times of more thermally sensitive types of malignancies which exhibit poor vascularity, usually an advantage for important medical reasons.
In this regard, researchers have commonly reported that as a consequence of these thermal characteristics of most malignant growths and the thermal sensitivity of normal body cells, hyperthermia temperatures for treatment of human cancer should be carefully limited within a relatively narrow effective and safe temperature range. Below a threshold temperature of about 41.5.degree. C. (106.57.degree. F.), significant thermal destruction of most malignant growths of cells has normally not been found to occur.
At slightly higher hyperthermic temperatures, above the approximate range of 43.degree. C. to 45.degree. C. (109.4.degree. F. to 113.degree. F.), thermal damage to most types of normal cells is routinely observed; thus, great care must be taken not to exceed these temperatures in healthy tissue. Exposure duration at any elevated temperature is, of course, an important factor in establishing extent of thermal damage to healthy tissue. However, if large or critical regions of the human body are heated into, or above the, 43.degree. C. to 45.degree. C. range, for even relatively short times, serious permanent injury or death may be expected to result.
Historically, alternating electric currents, at frequencies above about 10 KHz, were found late in the last century to penetrate and cause heating in biological tissue. As a result, high frequency electric currents, usually in the megahertz frequency range, have since been widely used for therapeutic treatment of such common bodily disorders as infected tissue and muscle injuries. Early in this century, the name "diathermia" was given to this EMR tissue heating technique, and several discrete EMR frequencies in the megahertz range have subsequently been allocated specifically for diathermy use in this country by the Federal Commerce Commission (FCC).
A number of even more current discussions relating to EMR hyperthermia treatment of cancer may, for example, be found in a compilation of articles on the subject published in the book "Cancer Therapy by Hyperthermia and Radiation", edited by Christian Streffer et al and published by Urban and Schwarzenberg; Baltimore, Munich 1978.
In spite of there having been reported encouraging and often apparently successful medical results obtained by using EMR induced hyperthermia to treat malignant growths in humans, the treatments have normally been of an experimental nature, typically being used on cancer patients otherwise considered incurable or terminal, since serious problems relating to hyperthermic damage to healthy tissue have commonly been encountered. As with conventional surface heating, these healthy tissue damage problems are particularly associated with thermally destroying malignant growths deeply located in, or close to, thermally sensitive tissue.
This unintended EMR thermal damage of healthy tissue can typically be attributed to design and use of existing EMR irradiating apparatus, rather than to any basic deficiency in the concept of EMR hyperthermia treatment. EMR apparatus used, for example, often radiate excessive and/or improperly controlled EMR heating fields. A further disadvantage is that the specific diathermy allocated frequencies which are ordinarily used are typically non-optimum radiating frequencies for deep penetration. In addition, existing EMR hyperthermia apparatus and techniques tend to increase incidence and severity of thermal "hot spotting" in healthy tissue, as may be caused by uncontrolled constructive interference of applied energy waves, either by characteristic reflections at interfaces between different types of tissue, or by simultaneous use of more than one EMR applicator.
To overcome these and other problems associated with heretofore available EMR hyperthermia apparatus used to medical research or other medical purposes, applicant has disclosed improved EMR hyperthermia apparatus in U.S. patent applications, Ser. Nos. 002,584, now U.S. Pat. No. 4,271,848, and 048,515, now U.S. Pat. No. 4,341,227, filed on Jan. 11, 1979 and June 14, 1979, respectively. In these two patent applications, parallel plate and waveguide-type EMR applicators, together with associated EMR systems, were described and claimed, the applicators being particularly adapted for irradiating biological tissue or tissue simulating matter from outside the tissue. Emphasis was placed on broad band EMR capabilities, enabling, for example, research definition of important parameters associated with hyperthermic treatment of malignancies in humans. Also described in such patent applications was simultaneous operation of two (or more) applicators arranged to improve deep tissue heating characteristics.
In applicant's subsequent U.S. patent application, Ser. No. 050,050, filed on June 19, 1979, now abandoned, needle-type invasive EMR applicators, for enabling EMR hyperthermia in sub-surface tissue regions, were described. By surrounding, with a phased array of these invasive applicators, a localized tissue region, such as a region containing a malignant growth, substantially uniform heating of the surrounded region by constructive interferences of the synchronous EMR filed was described.
However, there still exists an important need for EMR hyperthermia apparatus capable of causing uniform deep EMR heating of thick tissue masses, such as trunk and thigh portions of an adult human body, in which large or widely dispersed malignant growths may be found. For these and similar regions of the body, an encircling annular EMR Applicator apparatus, which may comprise an array of smaller applicators, is odinarily preferred so that EMR energy is emitted inwardly from all around the enclosed body region to be EMR heated.
To this end, large annular magnetic coils have been used to radiate a magnetic field into a body region disposed through the coil. Although such radiated magnetic field are known to penetrate deeply in human tissue, uniform heating across the encircled tissue region is normally not possible. This has been reported by several researchers. This is because the induced currents couple much stronger in the longer current paths along the outer tissue regions than in the center. Thus, when such annular magnetic coils are used to cause hyperthermia in biological tissue, near surface tissue regions can be expected to be heated much more than underlying central tissue regions.
Presently available EMR applicators have failed to permit deep heating of a large target, such as a human torso, without severely damaging the overlying tissue. This is due to the lossy nature of biological tissue, which absorbs a certain fraction of EMR passing therethrough, such fraction being dependent on the tissue type and frequency of the EMR. Large amounts of the available power are absorbed near the surface, leaving relatively small amounts to be absorbed in the deeper regions. This causes excessive tissue heating at the surface, with little therapeutic heating in the desired location, which is the central region of the target.
Some heating improvement can be had in the central heating region by using several applicators surrounding the target. Such an array typically energizes the individual applicators at different frequencies, or frequencies which vary slightly with time, in order to avoid undesired hot regions caused by constructive interference between the different wavefronts. Such arrangements increase the power density in the central region to approximately the sum of the power densities due to the individual applicators without increasing appreciably the energy absorbed in a unit area of the target's surface. However, the power density in near-surface regions is usually still such as to be harmful to biological targets before heating reaches therapeutic values in the deeper regions.
When success in thermally treating deeply located malignant growths by heretofore available types of annular applicator apparatus has been experienced, the results appear to be more attributable to poorer heat dissipation properties of large malignant growths than to the desirable uniformity of heating. Heating uniformity, not heretofore available, would also enable effective thermal treatment, for example, of deeply located, widely dispersed, small groups of malignant cells in early stages of growth before the associated reduced heat transfer characteristics become significant. Ability to provide at least substantially uniform heating of encircled tissue regions is also very important, for example in areas of EMR hyperthermia research into hyperthermic effect on normal healthy tissue.